Jrome, I use SED (the original Svetlana not the new ones made by Sovtek?) KT88's and 6550C's in several different amps. Each tube type sounds better in the amps that were designed for them in the first place.
When I put the KT88 in an amp that was designed for a 6550 the sound is bloated/warm thru the mid bass/lower mids, rolled in the highs, and looses some clarity. It can sound lovely if you are not too uptight, or the sound better matches your other components, including the small tubes in the amp. Whereas the 6550C is more tonally correct - tight bass, transparent mid's, etc.
When I put a 6550C into an amp designed for a KT 88 it starts to sound like a SS amp. Bright, flat, very cool thru the mids and upper bass. To counter this I find I must use a small tubes noted for warmth.
As a matter of personal choice, and a general observation, I find I enjoy the sound of an amp designed around the KT88 more than the 6550.
Now comes the fun - whichever one you decide to go with, I'll bet you end up playing with the small tubes. Thats where much of the real magic is, matching the power tubes and the small tubes.